The invention relates to perfluorodiacylperoxides used as polymerization initiators in a wide temperature range.
Specifically the invention relates to perfluorodiacylperoxides obtainable with good yields from the respective perfluoroacyifluorides and having a high hydrolytic stability, in particular when used as polymerization initiators in aqueous medium. The high hydrolytic stability of said initiators confers to the polymerization process improved yields while the perfluorinated structure of the initiator allows to obtain polymers having stable perfluoroalkyl end groups.
The use as polymerization initiators of halogenated diacylperoxides is known in the prior art. These compounds can be obtained by synthesis from the respective acyl-halides, using various methods described in the literature. The perfluorodiacylperoxide synthesis from the respective perfluoro-acyl-halides in the presence of H2O2 and NaOH is described, for example, by H. Sawada in Chemical Review, 1996, Vol. 96, 1779-1808. In the prior art to synthesize perfluorodiacylperoxides, perfluoroacylchlorides are preferably used since they are more reactive than the corresponding perfluoroacylfluorides. In fact, perfluoroacylfluorides determine lower synthesis yields than the corresponding chlorides. Besides, the obtained perfluorodiacylperoxides cause secondary reactions since they are easily hydrolyzed by the reaction medium wherein they are produced.
The perfluorodiacylperoxides exemplified in the prior art as polymerization initiators give low yields when used in polymerization processes in aqueous medium. To overcome this drawback one can polymerize in the presence of solvent. However the polymerization in solvent is more expensive than that in aqueous medium. Besides, it is necessary to use particular solvents which have no environmental impact, otherwise supplementary units would be necessary for their treatment. Besides, the polymerization in aqueous medium allows to generally obtain higher polymerization yields and a better control of the molecular weights of the polymers.
From the industrial point of view it is therefore advantageous to polymerize in aqueous medium using initiators which are not hydrolyzed in the reaction medium. On the other hand non fluorinated initiators are known, for example ammonium persulphate, which have a good hydrolytic stability, thus allowing the water polymerization with good yields. However these initiators have the drawback to lead to the obtainment of polymers having unstable end groups and therefore unusable in applications where a high stability and a high purity degree of the final polymer are required, for example in optical applications. For these uses it would be necessary to treat them to obtain stable end groups.
In GB 781,532 it is described the polymerization of perfluoro-olefins using perfluorinated or chlorofluorinated solvents in the presence of initiators. In said patent it is pointed out how the use of perfluorinated peroxides as polymerization initiators in aqueous medium is not possible since the presence of water determines the deactivation of the peroxide itself already at temperatures higher than 0xc2x0 C.
U.S. Pat. No. 3,671,510 points out how the perfluorodiacylperoxides are hydrolyzed in the presence of water, wherefore the initiator efficiency is strongly compromised and therefore the monomer conversion is notably reduced. To overcome these inconveniences, this patent suggests the use of (perchlorofluoro)-di-acylperoxides as polymerization initiators in aqueous medium. However, the use of said peroxides implies the drawback to give polymers having unstable end groups with the above disadvantages.
EP 606,492 refers to a polymerization process in aqueous phase of tetrafluoroethylene with hexafluoropropene, wherein difluoroacylperoxides are used as polymerization initiators in the presence of suitable perfluoroalkylcarboxylic acids. Said perfluoroalkylcarboxylic acids decrease the tendency to hydrolysis of perfluoroacylperoxides. However, by operating in acid medium there is the drawback to have a poor stability of the obtained polymer latex.
The easiness to hydrolysis of perfluorodiacylperoxides is more stressed for the compounds having a low molecular weight, see Sawada et al in Chem. Abs. Vol. 112:117996b and patent application WO 97/08142. Said patent application describes a synthesis method to limit the tendency to hydrolysis of the obtained perfluorodiacylperoxides. The process variables, for example reaction temperature, contact time among reactants, mixing and ratio among reactants, must be very carefully determined and strictly maintained constant during the process. To maintain these variables in the narrow range useful to avoid hydrolysis, one uses complicated equipments, as ultrasound systems or micro-ejectors The drawback of the described process resides in that it is difficult to bring it on an industrial scale.
The Applicant has surprisingly found perfluorodiacylperoxides having end groups with branched or cyclic structure, which are not hydrolyzed in aqueous medium for long times at the corresponding temperatures of thermal decomposition. Depending on their decomposition temperature they are usable as polymerization initiators in a wide temperature range, from xe2x88x9220xc2x0 C. to 100xc2x0 C., and they allow to obtain improved yields (see the comparative Examples).
An object of the present invention are therefore perfluorodiacylperoxides having the following structures: 
wherein:
when Rf is F, Rfxe2x80x2, Rfxe2x80x3 are both xe2x80x94CF3;
when Rf is xe2x80x94CF3Rfxe2x80x2, Rfxe2x80x3 are C1-C3 linear or branched perfluorooxyalkyl groups; 
wherein:
Rv is selected from F, perfluorooxyalkyl, C1-C3 linear or branched perfluoroalkyl;
X1, X2 are selected from F, perfluoroalkyl, C1-C3 linear or branched perfluorooxyalkyl. 
wherein: p1 n=1-3
X3 is selected from F, C1-C3 linear or branched perfluoroalkyl, with the proviso that for n=3, X3 cannot be F; said perfluorodiacylperoxides meet the following condition: the thermal decomposition constants Kd (secxe2x88x921) in the presence of water do not undergo substantial variations with respect to the thermal decomposition constants in absence of water.
This means that the perfluorodiacylperoxides of the invention can be used as initiators in aqueous polymerization without any variation of the half-life times determined under anhydrous conditions. Therefore the invention compounds can be used also after storage in aqueous emulsion for long times, even over 2-3 days, without undergoing any hydrolytic decomposition. This represents a further advantage from the industrial point of view since there is no need of immediate use of the acylperoxide differently from what happens for the acylperoxides known in the prior art.
The perfluorodiacylproxides of the invention are obtained by synthesis of the corresponding perfluoroacylhalides in the presence of H2O2 and NaOH. In particular, they can be prepared by addition of perfluoroacylfluorides to a biphase system formed by a halogenated solvent and by an aqueous solution containing H2O2 and an alkaline metal hydroxide, such as for example NaOH, KOH. The halogenated solvent can be 1,1,2-trichloro-1,2,2-trifluorethane, trichlorofluoromethane, perfluoroheptane, perfluoropolyethers. The fed amounts of perfluoracylfluoride range from 0.5 to 2 moles per mole of H2O2; the amount of alkaline metal hydroxide is in the range 0.8-1.5 moles per mole of perfluoroacylfluoride. The system is maintained under stirring in a temperature range from xe2x88x9215xc2x0 C. to 20xc2x0 C., preferably from xe2x88x925xc2x0 C. to +5xc2x0 C. The organic phase containing the reaction products in solution is recovered from the aqueous phase by separation. A washing step with water of the organic phase and subsequent anhydrification with sodium sulphate follows.
A further object of the present invention is a polymerization process in solvent or in aqueous medium of one or more fluorinated monomers wherein as polymerization initiators the above perfluorodiacylperoxides are used.
As polymerization solvents, fluorinated solvents, preferably (per)fluoroalkanes or (per)fluoropolyethers are used.
As said the compounds of the invention can be used in a wide temperature range depending on the polymerization type. This is possible since the perfluorodiacylperoxides of the invention decompose in a surprisingly wide temperature range. For example, in the preparation of vinylidene fluoride (VDF)-based polymers it is preferable to use low polymrization temperatures (0xc2x0-10xc2x0 C.). Viceversa, in the case of copolymers having a high content of not very reactive mono-mers, as in the case of copolymers of 2,2,3-trifluoro-4-tri-fluoromethoxy-1,3-dioxole (TTD), it is preferable to carry out the polymerization at higher temperatures (higher than 50xc2x0 C.).
In particular for the polymerization reactions at temperatures of the order of 50xc2x0-80xc2x0 C., the compounds of structure (C) or the compound of structure (A) having the formula: 
are preferably used.
For the polymerization ractions at low temperature, of the order of xe2x88x9220xc2x0-+25xc2x0 C., the compounds of structure (A) of formula: 
are preferably used, wherein when Rf is xe2x80x94CF3, Rfxe2x80x2 and Rfxe2x80x3 are C1-C3 linear or branched perfluorooxyalkyl groups. Said acylperoxides of group (A) can furthermore be used as polymerization initiators in solvent at a temperature lower than 0xc2x0 C.
As fluorinated monomers polymerizable in the presence of the perfluorodiacylperoxides of the invention, we can mention:
C2-C8 perfluoroolefins, such as tetrafluoroethylene (TFE), hexafluoropropene (HFP);
C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, CH2xe2x95x90CHxe2x80x94Rf perfluoroalkylethylene, wherein Rf is a C1-C6 perfluoroalkyl, hexafluoroisobutene;
C2-C8 chloro-fluorolefins, such as chlorotrifluoroethylene (CTFE);
CF2xe2x95x90CFORf (per) fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per) fluoroalkyl, for example CF3, C2F5, C3F7;
CF2xe2x95x90CFOX (per)fluoro-oxyalkylvinylethers, wherein X is: a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per)fluorooxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl;
perfluorodioxoles, such as 2,2,4-trifluoro-5-trifluorome-thoxy-1,3-dioxole (TTD), 2,2-bis-trifluoromethyl-4,5-di-fluoro-dioxole (PPD);
sulphonic monomers, such as for example CF2xe2x95x90CFOCF2CF2SO2F;
fluorinated dienes as for example CF2xe2x95x90CFOCF2CF2CFxe2x95x90CF2, CF2xe2x95x90CFOCCl2CF2CFxe2x95x90CF2, CF2xe2x95x90CFOCF2OCFxe2x95x90CF2, CF2xe2x95x90CFOCF2OCClxe2x95x90CF2, CF2xe2x95x90CFOC (CF3)2OCFxe2x95x90CF2.
The polymerization in aqueous medium can be carried out in suspension, in emulsion or in microemulsion in the presence of the perfluorodiacylperoxides of the invention acting as initiators. The initiator feeding procedures can be in a continuous way or by a single addition at the starting of the polymerization. The amount of perfluorodiacylperoxide initiator is in the range 0.0001%-5% by moles with respect to the amount of the fed monomers. The polymerization temperature can be in the range from xe2x88x9220xc2x0 C. to 80xc2x0 C. at pressures comprised between 2 and 50 bar.
In the case of polymerization in aqueous emulsion, the presence of a surfactant is necessary, the fluorinated surfactants such as perfluorooctanoate or mixtures of ammonium, potassium or sodium perfluorooctanoate, perfluorononanoate, perfluorodecanoate are particularly preferred. It is particularly suitable to carry out the polymerization in aqueous phase in the presence of perfluoropolyethers as surfactants. Said perfluoropolyethers can be added to the reaction medium under the form of microemulsion, as described in U.S. Pat. No. 4,864,006.
The present invention will now be better illustrated by the following embodiment Examples, which have a merely indicative purpose but not limitative of the scope of the invention.